Field
This application relates generally to wireless communication and more specifically, but not exclusively, to allocating timeslots, and optionally radio frequencies, for the transmission of beacons.
Introduction
A wireless communication network may be deployed over a defined geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, macro access points (e.g., that provide coverage via one or more macro cells) are distributed throughout a network to provide wireless connectivity for access terminals (e.g., cell phones, portable devices, etc.) that are operating within the geographical area served by the network.
As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance. To supplement conventional network access points (e.g., macro access points), small-coverage access points (e.g., with transmit power of 20 dBm or less) may be deployed to provide more robust coverage for access terminals. For example, a small-coverage access point installed in a user's home or in an enterprise environment (e.g., commercial buildings) may provide voice and high speed data service for access terminals supporting cellular radio communication (e.g. CDMA, WCDMA, UMTS, LTE, etc.).
Conventionally, small-coverage access points may be referred to as, for example, femtocells, femto access points, home NodeBs, home eNodeBs, or access point base stations. Typically, such small-coverage access points are connected to the Internet and the mobile operator's network via a DSL router or a cable modem. For convenience, small-coverage access points may be referred to as femtocells or femto access points in the discussion that follows.
Applications running on an access terminal may make use of the location (e.g., position) of the access terminal. For example, the location of an access terminal may be reported during a 911 call by the access terminal. As another example, an access terminal-based navigation system uses the current location of the access terminal for providing navigational aids.
The location of an access terminal may be determined relative to a set of access points, whose locations are known. Two techniques for determining the location of an access terminal include a time-based approach and signal strength-based triangulation.
The time-based approach relies on estimating the distance between two communication points by estimating the difference in the signal propagation delay between them. The accuracy of the time-based approach is limited, however, by signal propagation characteristics that tend to be location dependent (due to multipath effects and lack of a line of sight component). Furthermore, the resolution in a time-based approach depends on the chip duration over which a propagation delay change can be observed (which can be about 40 meters).
In a signal strength-based triangulation method, an access terminal may measure the pathloss between a set of access points and itself. The pathloss values are used for estimating the distance between each access point and the access terminal. For example, a pathloss fingerprint corresponding to the set of path loss values is compared with a previously defined database of pathloss fingerprints (each of which corresponds to a particular location) to determine the current location of the access terminal. The accuracy in estimating the location improves with the number of distinct access points from which the pathloss values are calculated. The database of pathloss values can be generated by either developing mathematical models and simulating them or by using technician walks. The simulation-based approach may be more suitable for a large enterprise setup, while technician walks may be used to correct any errors in the modeling.
In the case of signal strength-based triangulation, the pathloss values may be calculated from Ecp/Io and the Io values of beacons transmitted by the access point as measured and reported by the access terminal to its serving access point. Here, Ecp is the received signal strength of a beacon at the access terminal and Io is the total received energy at the access terminal including noise. The ratio Ecp/Io is therefore the received signal to interference-plus-noise ratio (SINR) of the beacon at the access terminal. The serving access point can calculate the pathloss between access terminal and the access points that transmitted the beacons using knowledge of the transmit powers of the access points. In cdma2000, the access terminal (i.e., a mobile station) can use the pilot strength measurement message (PSMM) reports for sending the signal strength values measured on the femtocell frequencies, and the candidate frequency search (CFS) messages for signal strength values measured on macro frequencies.
To measure and report beacon signal strength, the SINR of the signal at the access terminal should be higher than a detection threshold (e.g., −16 dB). When an access terminal is close to its serving access point, the interference from the serving access point may overwhelm the beacons received from the non-serving access points, thereby degenerating the triangulation set to just one. While this can be mitigated to some extent by having a separate beacon channel, beacons from multiple access points may still overlap, making it difficult for the access terminal to decode them and measure their strengths individually. This can affect the accuracy of the location fix. Consequently, a need exists for more effective techniques for determining the location of an access terminal.